1. Field of the Invention
The invention relates to a circuit arrangement comprising: input terminals for deriving a supply current from a supply source, means I for generating a control signal S, means II provided with a converter which is fitted with at least one switching element and with control means which trigger said switching element with high frequency in a manner which is dependent on the value of the control signal S, means III for generating a voltage Sc which is a measure for an instantaneous value of a supply voltage delivered by the supply source and output terminals coupled to the means II for connection to a light source.
2. Description of Related Art
A circuit arrangement of a kind described in the opening paragraph is known from European Patent Specification EP 507 393, corresponding to U.S. Pat. No. 5,196,768. The known circuit arrangement, when connected to a supply source which delivers a sinusoidal supply voltage, draws a supply current of approximately corresponding shape. The means III of the known circuit arrangement is formed by a rectifier circuit. An up-converter is operated by means of the voltage generated by the rectifier circuit. The control signal is generated by detection means which measures a charging current of capacitive means which is supplied by the up-converter. Such a circuit arrangement may serve for supplying a semiconductor light source.
The comparatively high luminous efficacy, of the order of 15 lm/W, and the long life, a few tens of thousands of hours, of semiconductor light sources render them attractive for use as traffic lights. At the moment, traffic lights are usually constructed as incandescent lamps. Solid state relays (SSRs), provided with a TRIAC switching element and a control circuit, are mostly used for switching traffic lights. The SSRs operate reliably at the comparatively high loads, of the order of 150 W, of the incandescent lamps used.
If a semiconductor light source is used as a traffic light, however, a much smaller load, of the order of 15 W or less can suffice. It may happen that the TRIAC does not enter a conducting state when such a semiconductor light source is operated in conjunction with a known circuit arrangement and an existing SSR. A supply current drawn from the SSR in that case, flows mainly through the control circuit and may damage the latter.